1. Field of the Invention
The present invention relates to an optical system. More particularly, the present invention relates to an optical system suitable for use in an optical apparatus, such as a silver-halide film camera, a digital still camera, a video camera, a telescope, a binocular, a projector, or a copying machine.
2. Description of the Related Art
An optical system used in an optical apparatus, such as a digital camera, a video camera, or a projector, is required to have a short overall lens length (i.e., a length from a first lens surface on the object side to an image plane) corresponding to reduction in size of the optical apparatus. However, as the overall lens length reduces, various aberrations tend to increase. In particular, axial chromatic aberration and transverse chromatic aberration are increased, thus resulting in deterioration of optical performance.
Known techniques for reducing chromatic aberrations of an optical system include a method of using an anomalous partial dispersion material as an optical member and a method of using a diffractive optical element having a diffractive action (see U.S. Pat. Nos. 6,115,188, 7,136,237, 7,193,789, 6,381,079, 7,426,083, and 7,253,973).
In U.S. Pat. No. 6,115,188, the chromatic aberrations are satisfactorily corrected by using a material having anomalous partial dispersion at a low level of dispersion, such as fluorite, to form a positive lens, and by using a high dispersion material to form a negative lens.
In U.S. Pat. Nos. 7,136,237 and 7,193,789, the chromatic aberrations are satisfactorily corrected by using, as the anomalous partial dispersion material, a fine-particle dispersed material that is prepared by mixing fine particles, such as ITO or TiO2, in a resin material, or a resin material having an anomalous partial dispersion characteristic.
To considerably correct the chromatic aberrations, which are increased when the overall lens length is reduced, by using the lens made of the low dispersion material, such as fluorite, refractive power of the lens surface has to be changed to a large extent. It is therefore important to appropriately set the refractive power and to arrange the lens surface at an appropriate position in the optical system. If the refractive power and the arranged position of the lens surface are inappropriate, a difficulty arises in correcting various aberrations, such as spherical aberration, coma aberration, and astigmatism, while realizing correction of the chromatic aberrations.
The diffractive optical element has a very small absolute value as a numerical value corresponding to the Abbe number. Accordingly, just by slightly changing diffractive power (inverse number of the focal length), the chromatic aberrations can be largely changed without substantially affecting the spherical aberration, the coma aberration, and the astigmatism.
In U.S. Pat. Nos. 6,381,079, 7,426,083, and 7,253,973, the chromatic aberrations are corrected by utilizing a negative dispersion characteristic and strong anomalous dispersion of the diffractive optical element.
However, if there is diffracted light at unnecessary orders of diffraction other than desired imaging light, the diffracted light becomes flare light that largely deteriorates the image-forming performance. For example, if a light source with high brightness is present in an object, a flare due to the unnecessary diffracted light appears around the light source. Also, if strong light coming from the outside of a frame, e.g., the sunlight, enters the diffractive optical element, a flare is generated and contrast of the entire frame is reduced. For that reason, when the diffractive optical element is used, it has to be arranged at an appropriate position in the optical system with appropriate diffractive power.